Ship & Boat International: eNews March/April 2019
Finnish towage and salvage operator Alfons Håkans has retrofitted one of its ASD tugs with bronze propellers, in readiness for a new development in icebreaking operations, writes David Tinsley. The recipient tug, the 26m x 9m Calypso, has been modified to push a detachable icebreaking bow designed to improve winter navigation on the Saimaa Canal and Lake Saimaa waterway system in eastern Finland.
The four-bladed bronze propellers were manufactured by Finnish company TEVO and designed in accordance with Aker Arctic Technology’s new propeller strength dimensioning criteria. Although the tug was built to 1A ice class, the bronze-cast propellers are suitable for vessels constructed to 1A Super ice class, corresponding to the PC6 polar standard. The bronze propeller is the result of a three-year R&D project jointly conducted by TEVO, Aker Arctic Technology and the Technical Research Centre of Finland (VTT).
Although not as strong a material as stainless steel, bronze offers better corrosion resistance, incurs lower cost and is easier to manufacture and maintain. While bronze is widely used for open-water propellers, the cooperation partners were keen to establish its feasibility in high ice-class vessel applications. The studies included trials on a Finnish multipurpose response vessel operating in the Baltic, involving ahead/astern tests in level ice of 600-850mm thickness and 6m-thick ice ridges. For the trials, one of the ship’s two stainless steel propellers was substituted by the prototype bronze propeller. The bronze unit was shown to generate 18% more thrust than the stainless steel propeller at the same power.
The bronze propellers specified for Calypso were cast at TEVO’s Turenki foundry and fitted during the tug’s drydocking at Western Shipyard in Teijo, Finland, during November 2018. They were incorporated into the existing Aquamaster US205-type azimuthing propulsion units minus the original nozzles. The susceptibility of the nozzles to clogging during icebreaking operations is avoided with the new, nozzle-free propellers.
Calypso is currently continuing her ship-handling role at ports in southern Finland, and will move north to the Saimaa system once the motorised, removable bow has been fitted, towards the end of 2019. The tug’s modification and scheduled redeployment followed the award by the Finnish Transport Infrastructure Agency of a 10-year contract to Alfons Håkans to perform icebreaking on the Saimaa inland waterway system using the detachable bow. Calypso and the innovative, removable bow will break a wider channel than that currently achieved, enhancing navigational conditions and capacity throughout the system.
Developed by the Agency and Finnish marine technical consultancy ILS, and model-tested by Aker Arctic Technology, the detachable bow will be 25.3m-long and 12.6m-wide. It will have its own propulsion system, to enhance manoeuvrability and efficiency, whereby a shaftline on either side of the bow will deliver 600kW of power. Construction has been entrusted to the BLRT Group of Lithuania, and the marrying of the unit with the tug will take place at BLRT’s Turku repair yard in Finland.